Themes > Science > Zoological Sciences > Animal classification > Polygenetic Tree of Kingdom Animalia > Protostome and Deuterostome Conditions > Introduction of Protostomes


The phylum Mollusks means "soft bodied". All mollusks are bilateral, and have a mantle, a tissue fold that hangs skirtlike over the body. They have gills, fleshy foot and many have a tonguelike, toothed organ that shreds food destined for the gut.

There are three classes, the cephalopods, gastropods, and bivalves.

The gastropods ("belly foots") are the largest class. These are snails or slugs with no shell.

The bivalves ("two-valved shell") consists of clams, scallops, oysters and mussels. All bivalves gills function in collecting food and in respiration. Water moves through the mantle cavity and the mucus on the gills traps the food.

The cephalopods ("head-foot") consist of squids, cuttlefish and octopuses. They move through jet propulsion.

They are the only mollusk with a closed circulatory system and have the largest brain relative to their body size.

The octopus is the most intelligent. It can problem solve and show emotions.

The phylum Annelid ("ringed body") are bilateral, segmented worms and leeches having a complex organ system.

The phylum Arthropod is the most successful organisms on earth. 80% of all animals have exoskeletons.

Six adaptations contributed to the success of arthropods are:

 

  1. Hardened exoskeleton. Arthropods have a cuticle that serves as an external skeleton, or exoskeleton. An exambple would be crabs and lobsters have a hardened exoskeleton. Arhtropods grow in puts by molting. By doing this, they shed their old exoskeleton while growing a new one.
  2. Specialized segments. Body segments became few as the arthropods evolved. The body segments fused or were grouped together such as in spiders, the segments combined into a forebody and hindbody.
  3. Jointed Appendages. Arthropods have jointed appendages (Arthropod means "jointed foot"). The appendages are used for feeding, locomotion, transferring sperm to females, or spinning sillk.
  4. Respiratory structures. Aquatic arthropods rely on gills for gas exchange. Air tubes evolved among insects.
  5. Specialized Sensory Structures. Eyes and other sensory organs help to make the Arthropod a success.
  6. Division of labor in the life cycle. They first develop as sexually immature larvae that molt and change as they grow. The larvae enter a pupal stage. Growth and transformation of larva into the adult is called metamorphosis.
The classes of Arthropods are:
  1. Crustaceans - Examples are blue crabs, lobsters. They repeatedly replace their exoskeletons by molting during growth.
  2. Arachnids - Examples are spiders, ticks, mites, blackwidow. Spiders and their relatives have a variety of appendages specialized for predatory or parasitic life styles.
  3. Centipedes - Have a long, segmented body with many legs.
  4. Millipedes - Have a rounded body with many legs
  5. Insects - Dominate earth with mammals


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